The present invention relates to high speed image constructing unit for a nuclear magnetic resonance imaging system and performs an image construct operation using two-dimensional or three-dimensional Fourier transformation methods.
A nuclear magnetic resonance imaging process is performed to obtain an image of a section, etc. of an object as follows. When an atomic nucleus (a hydrogen atomic nucleus, etc.) is put in an even static magnetic field, the spin of the atomic nucleus starts precessing around the static magnetic field. Then, a radiofrequency pulse (an RF pulse) having a frequency equal to that of the precession is applied to the object to induce nuclear magnetic resonance phenomena, thereby generating signals which can be used to form a target image. In the Fourier transformation method, each atomic nucleus in an object is made to repeat resonance and relaxation at a different frequency depending on the magnitude of the magnetic field by superposing in the static magnetic field an oblique magnetic field whose magnitude is variable in the specific direction. Thus, the resultant signal is processed by the Fourier transformation to analyze the frequency and construct an image.
In reference to the Fourier transformation method, there is a Fourier transformation Zeugmatography method (the original method) as a well-known example (refer to Kumar. A, Welti. D, and Ernst. R, "NMR Fourier Zeugmatography", J.Magn,Reson., 18, p.69 (1975)).
Another prior art technology is a Spin Warp Method (refer to Edelstein, W. A., Hutchison, J. M. S., Johnson, G & Redpath, T. W. "Spin Warp NMR Imaging and Applications to Human Whole-Body Imaging" Phys. Med. Biol, 25, P751 (1980)). In this technology, a field echo method is used as a mechanism for generating an echo.
FIG. 1 shows a block diagram of a common nuclear magnetic resonance imaging system to which the above described method is applied.
That is, a test object 1 is put in a static magnetic field coil 2 for generating an even static magnetic field. A computer system 11 comprises a pulse sequencer 13, a CPU 14, a hard disk 15, a console 16, a digital input interface 17, and an image construct operating unit 18, etc. They are connected to one another through a system bus 12.
When the console issues a pickup instruction, the CPU 14 activates the pulse sequencer 13. The pulse sequencer 13 controls at predetermined timing, an oblique magnetic field power source 5, 6, 7 (oblique magnetic fields Gx, Gy, and Gz are induced in the directions X, Y, and Z respectively), and an RF transmission system 8. Therefore, an oblique magnetic field coil 3 and an RF coil 4 generate a predetermined oblique magnetic field and a radiofrequency magnetic field respectively. Consequently, the test object 1 generates an NMR (nuclear nuclear magnetic resonance) signal. After being received by the RF coil 4, the NMR signal is tested and amplified by an RF receiving system 9. Then, it is transformed to a digital signal by an A/D converter 10, and is read into the computer system 11 through the digital input interface 17. The read data are sent to the image construct operating unit 18 and processed by an image constructing operation. Then, they are displayed as an image on the console 16. In FIG. 1, 12 is a system bus, and 15 is a hard disk for storing images.
FIG. 2 shows an example of data collecting timing applied to the two-dimensional Fourier transformation. The figure shows each output of the RF magnetic field, the oblique magnetic fields Gz, Gx, and Gy, and the operation of the NMR signal and the A/D converter 10.
In FIG. 2, 201 is a 90.degree. RF pulse; 202 is an oblique magnetic field for selecting a slice; 203 is an oblique magnetic field for encoding a frequency; and 204 is an oblique magnetic field for encoding a phase. When an image is obtained in an L.times.M matrix, an NMR signal 205 is sampled for L points (206) by the A/D converter 10.
The above processes for 201-206 are operated M times at the repetition time T.sub.R intervals with the phase encoding oblique magnetic field changed each time. Thus obtained L.times.M point data are processed into two-dimensional image data by the image construct operating unit 18 through an image regenerating operation including the two-dimensional Fourier transformation.
FIG. 3 is an example of the image construct operating unit 18. Since numbers 1-17 refer to the same parts as those referred to by the same numbers in FIG. 1, the explanation is omitted here.
In FIG. 3, a host interface mechanism 181 is provided in the image construct operating unit 18; an instruction control mechanism 182 controls each mechanism in the image construct operating unit 18 according to an instruction from the host computer; a data memory mechanism 183 stores data; and an operating mechanism 184 performs an operation necessary for constructing an image. Each of the mechanisms is connected through a local bus 185, and connected to the system bus 12 through the host interface mechanism 181.
The NMR signal (raw data) collected by the A/D converter 10 at the timing 206 shown in FIG. 2 are accumulated in the data memory mechanism 183 through the digital input interface 17, the system bus 12, and the host interface mechanism 181. With the image construct operating unit used for the nuclear magnetic resonance imaging system, the two-dimensional Fourier transformation method is applied in which the first-dimension data process is performed immediately after or in parallel with each data collection, and the second-dimension data process is performed after all the data are collected. The first-dimension data process is performed on the raw data accumulated in the data memory mechanism 183 by the operating mechanism 184 immediately after each data collection.
After data is processed by the first-dimension data process (intermediate data), they are stored again in the data memory mechanism 183. Thus, after all the data collections are completed and the first-dimension data process of all the data is completed, the operating mechanism 184 performs the second-dimension data process on the intermediate data accumulated in the data memory mechanism 183, generates image data, and accumulates them in the data memory mechanism 183. The accumulated image data are transmitted to the console 16 through the host interface mechanism 181 and the system bus 12, and then displayed thereon.
FIG. 4 shows an example of common data collecting timing applied to the three-dimensional Fourier transformation method.
Like in FIG. 2, FIG. 4 shows each output of the RF magnetic field, the oblique magnetic fields Gz, Gx, and Gy, and the operation of the NMR signal and the A/D converter 10. 301 in FIG. 4 is a 90.degree. RF pulse; an oblique magnetic field 302 encodes a phase in the direction of the Z axis; an oblique magnetic field 303 encodes a frequency; and an oblique magnetic field 304 encodes a phase in the direction of the Y axis.
When an image data is obtained in an L.times.M.times.N matrix, an NMR signal 305 is sampled for L points (306) by the A/D converter 10. The above processes for 301-306 are operated M times at the repetition time T.sub.R intervals with the phase encoding oblique magnetic field 304 in the direction of the Y axis changed each time. Then, the M-time processes are repeated N times with the phase encoding oblique magnetic field 302 in the direction of the Z axis changed each time. Thus obtained L.times.M.times.N point data are processed into three-dimensional image data by the image construct operating unit 18 through an image constructing operation including the three-dimensional Fourier transformation.
FIG. 5 shows an example of an image construct operating unit used for the nuclear magnetic resonance imaging system in which the three-dimensional Fourier transformation method is applied. In this method, the first-dimension data process is performed immediately after or in parallel with each data collection, the second-dimension data process is performed immediately after a predetermined number of repetition of data collections or in parallel with the data collections and the first- and second-dimension processes are performed in parallel, and the third- dimension data process is performed after all the data are collected. In FIG. 5, numbers 1-18, and 181-185 refer to the same parts as those referred to by the same numbers shown in FIG. 3.
The number 19 in FIG. 5 refers to another image construct operating unit having the same configuration as the image construct operating unit 18. Numbers 191-195 refer to the same elements as the numbers 181-185.
The NMR signal (raw data) collected by the A/D converter 10 at the timing 306 shown in FIG. 4 are accumulated in the data memory mechanism 183 through the digital input interface 17, the system bus 12, and the host interface mechanism 181 in the image construct operating unit 18. The first-dimension data process is performed on the accumulated raw data by the operating mechanism 184 immediately after each data collection. The first-dimension data process outputs intermediate data which are temporarily stored in the data memory mechanism 183, and then accumulated in the data memory mechanism 193 through the host interface mechanism 181, the system bus 12, and the host interface mechanism 191 in the image construct operating unit 19.
After completing M repetition of the L-point data collections and the first-dimension data processes by the image construct operating unit 18, and after storing in the data memory mechanism 193 the intermediate L.times.M-point data necessary for the second-dimension data process, the operating mechanism 194 performs the second-dimension data process on the intermediate data accumulated in the data memory mechanism 193, and stores the processed data again in the data memory mechanism 193. During these processes, the image construct operating unit 18 continuously performs the first-dimension data process. Thus, the first- and the second-dimension data processes are performed in parallel with the data collections.
After completing all the data collections, and the first- and the second-dimension data processes on all the data, the operating units 194 perform the third-dimension data process on the data accumulated in the data memory mechanism 193, generate image data, and accumulate them in the data memory mechanism 193. The accumulated data are transmitted to the console 16 through the host interface mechanism 191 and the system bus 12, and then displayed thereon.
With the above described image construct operating unit used for the nuclear magnetic resonance imaging system in which the two-dimensional Fourier transformation system is applied, the first- and the second-dimension data processes are separately performed by only one operating unit, and the data must be transmitted before and after the data processing operations. Therefore, too much time is taken for the image construct.
Besides, with the image construct operating unit used for the nuclear magnetic resonance imaging system to which the three-dimensional Fourier transformation method is used, the first-, second-, and third-dimention data processes are separately performed by only one operating unit, and the data must be transmitted before and after the data are processed like in the two-dimensional-Fourier transformation method. Therefore, too much time is taken for the image construct.